Metal rolling process



Patented Oct. 31, 1939 PATENT OFFICE METAL ROLLING PROCESS Robert William Graham, Alcoa, '1enn., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application March 5, 193'1,

Serial No. 129,221

11 Claims. (c1. so-am This invention relates to the production of sheet metal, and especially to the rolling of 'thin sheets of metal of relatively great length. More specifically, it relates to a method of rolling 5 aluminum and its alloys to produce coiled-sheets or strips characterized by especially bright surfaces on one or-both' sides.

In producing aluminum sheet material of thicknesses between foil and plate, various. rolling in procedures and schedules have been employed. In general, sheet ingots of suitable size are first broken down by hot rolling to the desired hot mill gauge, which depends on the thickness desired in the final product. From this point 15 reduction is usually effected by cold rolling to produce either flat sheet or coiled sheet of the desired gauge and temper. When the product is desired in the hard or fully worked temper, it is cold rolled from the hot mill gauge to the final gauge without annealing, if possible. T1 the product is desired in the softest .or fully annealed con dition, it is similarly rolled and then suitably annealed. When the product is desired in some intermediate temper, however, the metal is rolled 25 to a gauge heavier than that desired in the final product, annealed, and subjected to enough additional cold rolling to produce the desired amount of strain-hardening. When producing fiat sheet, the sheet at the ho 3o mill gauge is cut into convenient lengths and passed repeatedly through a suitable mill, which may be a single stand of rolls or a train of roll stands. Twenty or more passes through the mill may be required for' the production of flat sheet, 35 and each pass involves a certain amount of handling and manipulation by the mill attendants. When producing coiled strip or sheet, however, the metal at the hot mill gauge is fed through a continuous mill which preferably comprises five or seven or some other suitable number of stands on which the metal is reduced in thickness with comparative rapidity. Suiiicient reduction is usually obtained in a single pass through the continuous mill, after which the sheet or strip 45 is coiled, either by hand or on a suitable coiler or coiling block. The coiled strip may sometimes be sold or utilized in the form in which it is obtainedfrom the continuous mill, or after fiattening it and cutting it to size, but is usually sub- 50 jected to further rolling or other treatment. It is generally given a few passes through a strip mill consisting of a single stand of rolls, and the strip is then re-coiled after each pass through this mill. It has also been the usual practice to reverse the metal between passes through the strip mill, so that the side of the metal which faces upwardly during one pass through the mill faces downwardly during the next pass. After reduction in the strip mill is completed, the coiled sheet may be given a finishing pass 5 in another mill, or may be annealed, with or without further rolling, to obtain the desired temper.

Flattened coiled sheet is considerably less expensive than the flat sheet previously mentioned, because its production is more economical and there is less scrap loss. However, in the past this product has not had a sufliciently smooth or bright surface for some purposes, and consumers requiring aluminum sheet possessing these characteristics have therefore had to employ the more expensive flat sheet. The principal object of my invention is to provide an economical and otherwise satisfactory process of producing coiled sheet or strip having one or both surfaces sufiiciently bright and smooth to permit its use' in applications which have heretofore required the product of the fiat rolling process. Another object is to provide coiled aluminum sheet or strip having surface characteristics such that after as flattening it may be employed for purposes which prevlously required ,use of the product known commercially as flat sheet. Other objects and advantages of my invention will become apparent from the following description.

From extensive. study of and experimentation with rolling processes and the factors affecting the results of such processes, I have found that one of the most important factors in the brightness of rolled sheet aluminum is the angle at :5 which the metal enters the rolls, especially dur ing the final orfinishing passes to which it is subjected. It appears that when a strip of metal is inserted between a pair of rolls at an angle other than that of a plane perpendicular to the 40 plane containing the axes of the rolls, the rolls bite on only one side of the strip, this being the outer or convex side of the curve resulting when the metal entering at an angle passes through the rolls. For example, when using rolls mounted in the usualvertical position and inserting the metal from a point above the horizontal, the lower side ofthe sheet is the outer or convex side and receives all or substantially all of the bite or action of the rolls. This outer side is burnished and acquires a polish, while the inner or concave side remains dull and may become even duller than it was before entering the rolls.

In referring to one side of the sheet as being bright or polished, I mean that it is specularly reflective. In other words, it possesses a mirrorlike surface, whereas the other side of the sheet, which has been described as being dull, in reality possesses a diffusely reflective surface which also reflects light. The distinction between specular and diffuse reflection is well recognized in the art of illumination. The discussion of the subject found in the book The Science of Seeing by M. Luckiesch and F. K. Moss (1937) pages 310, 315, and 316, will serve as an example of such recognition. The term brightness as it is herein employed refers to the specular quality of the sheet surface rather than to light reflection in general.

As previously noted, it has been the general practice in the production of coiled sheet to reverse the sheet between passes through the mill. As a result, even in those instances in the prior practice when the sheet may have entered the rolls at the proper angle for burnishing or brightening it on one side, this effect was neutralized or destroyed on the succeeding pass. One feature of my improved process is that when producing metal with one side bright the metal is fed to the rolls at a suitable angle, and is not reversed between successive passes through the rolls, at least during the final steps of its reduction. One side of the sheet therefore becomes continually brighter and brighter during these final passes. When this practice is followed, the angle of entry is not critical within any narrow limits, but good results have been obtained with an angle of entry of about 3. This angle may be maintained by properly setting the last bar in a bridle of known type, or otherwise. I prefer to have the sheet fed to the rolls at an angle above the horizontal, so that the lower side of the sheet is brightened. With a bridle bar adjusted to give a theoretical angle of entry of about 3", for example, the actual angle of entry may be somewhat greater than 3 because of the set resulting from weaving the strip through the bridle.

I have found that in order to produce sheet or strip material having both sides of substantially equal brightness the angle of entry must be substantially 0 (from the plane perpendicular to the plane of the roll axes) so that there is the same amount of reduction and consequently of burnishing or polishing on both sides of the strip. This requirement has been clearly demonstrated in practice, and suitable devices have been developed for insuring the entry of the metal into the rolls at an angle of substantially 0. For example, this may be accomplished by means of a special type of bridle, or by the use of a single bridle bar if it is properly set and suificient tension is applied to the metal being fed to the rolls. A larger number of passes at a given rate of reduction is required to produce sheet with two bright sides than sheet with one side of the same brightness.

In addition to the angle of entry, several other factors are important in the production of coiled sheet or coiled strip with one or two especially bright surfaces, in accordance with my invention. One of these factors is the finish of the rolls employed, particularly in the final pass or passes to which the metal is subjected. -A highly polished roll is required for at least the finishing pass on the side or sides of the sheet which are to be bright finished.

Several known roll lubricants are satisfactory for use in the production of bright strip by my process. In general, it has been found that a suitable lubricant must be light enough to introduce a burnishing action and prevent metal slippage, but still heavy enough to prevent coating the rolls and destroying their finish. The exact choice of lubricants for any particular op eration may be varied according to the temperature of the metal and the rolls, and the hardness of the metal.

I have also found that better results are obtained when rolling on warm or hot rolls than when using cold rolls, especially in the finishing passes. When rolling metal with one bright side, better results are obtained when a temperature differential is maintained between the two rolls. Good results are obtained when this differential is about 50 F., and I prefer to have the roll which contacts the bright side of the metal (usually the bottom roll) at a temperature of about F., and the other roll at a temperature of about 170 F. These temperatures may be maintained by heating or cooling, as may be required, with steam, hot water, or cold water, which are supplied to the rolls through suitable connections. When rolling strip with two bright sides, the roll temperatures should be about equal, and preferably in the neighborhood of 140 to 150 F. during the finishing pass or passes.

With other conditions equal, I have found that the brightness of the strip produced by my method varies inversely as the roll speed, possibly because at slow speeds the rolls smooth out minute wrinkles better than at high speeds. For example, exit speeds of 100 feet per minute gave better results than speeds of 150 feet per minute. In many instances, linear speeds of about feet per minute in the finishing pass give satisfactory results. v

I have found that the tension of the metal during rolling has little effect on the brightness of the strip produced, except that excessive tension may stretch soft metal and thereby cause wrinkling or dullness, which is, of course, undesirable.

I have also found that better results are obtained if the metal is warm before it enters the finishing pass. Sufllcient heat may remain in the metal from previous rolling passes, or may be developed during rolling when using light lubricants. In either case the temperature of the metal during the finishing pass should be at least 100 F., and preferably from 115 to F.

. No special rate of reduction seems to be necessary in the production of coiled strip or sheet with one or two bright sides, and the reductions ordinarily employed in the production of commercial coiled sheet give satisfactory results in my process. Reductions greater than those normally employed in commercial operation sometimes produce still brighter surfaces, however, probably because the heat generated under such circumstances is suflicient to burn off the lubricant and give the effect of rolling on a dry roll. Regardless of the reduction schedule employed, it is necessary to the production of satisfactory strip with one bright side that the same side of the metal be kept down during the last three or four passes.

The rolling procedure described hereinabove gives satisfactory results in theproduction of bright coiled strip or sheet in the hard temper or fully annealed temper, but further considerations are involved in the production of bright strip in the intermediate tempers. The standard practice for annealing coiled sheet has been to subject it to a furnace temperature of about 900 E, which was requiredto burn off the oil. When It bright strip produced as described hereinabove is subjected to this annealing treatment, a coating forms on the surface of the metal as a result of the oxidation occurring at the elevated teming improves the brightness again, but the effect of this annealing practice issuch that it is often discernible on sheet which has been annealed at a relatively heavy gauge and then rolled to a hard temper. I have found, however, that much less coating is formed if the bright'strip is annealed at lower temperatures, such as 650 F. or less,

which are permissibleafter cleaning the metal with a suitablesolvent to prevent staining. Sheet rolled after this-type of annealing is much brighter, and is quite satisfactory for my purposes-,even' in the intermediate tempers. 'Good results are also obtained by continuous annealing processes in which the temperature attained by the metal is relatively low and the metal is subjected to it for a relatively short time. Annealing ma nonoxidizing atmosphere is alsobeneficial in minimizing the formation of undesirable coatings.

By employing the process described hereinabove, I have been able to produce coiled sheet or strip aluminum in all tempers having surface characteristics such that it has been successfully used in applications which previouslynecessitated the use of more expensive flat rolled sheet. This result has been obtained at a cost not substantially greater than that formerly involved in the production'of commercial coiled sheet. It is to be understood that the term aluminum is used herein to denote aluminum of commercial purity, which may contain up to about 1% of impurities, as well as aluminum base alloys which contain higher percentages of alloying ingredients.

I claim:

1. The method of producing aluminum sheet material in coiled form of greater thickness than foil characterized by having one bright surface of substantially the brightness of aluminum sheet produced by flat rolling, which comprises subjecting a strip of cold rolled aluminum sheet material to a plurality of passes through a mill having vertically mounted, lubricated rolls to which the strip is fed at an angle to the center line of the pass and from the side of said center line opposite to that where the roll producing the bright surface is located, and without reversing the strip between successive passes, and maintaining the rolls at different temperatures, the roll having the lower temperature being in contact with the bright side of the sheet.

2. The method of producing aluminum sheet material in coiled form of greater thickness than foil characterized by having one bright surface of substantially the brightness of aluminum sheet produced byflat roiling, which comprises subjecting a strip of cold rolled aluminum sheet material to at least one pass through a mill having vertically mounted, highly polished rolls to which the strip is fed from a position above the horizontal plane passing between the rolls, and maintaining the rolls at temperatures between about and about 200 F. during said pass, the upper roll being maintained at a temperature higher than that of the lower roll.

3. The method of producing aluminum sheet material in coiled form of greater thickness than foil characterized by having one bright surface of substantially the brightness of aluminum sheet produced by flat rolling, which comprises sub- ,iecting a strip of cold rolled aluminum sheet material to at least ongpassthrough a mill having vertically mounted, highly polishedrolls to which. the strip is fed from a position above the horizontal plane passing between the rolls, and

maintaining the rolls at different controlled temperatures between about 100 and about 200 F. during said pass, the upper roll being maintained at a temperature about 50 F. higher than that of the lower roll.

4. In a method of j producing one side bright aluminum sheet material of greater thickness than foil, the sheet being characterized by one surface having substantially the brightness of flat rolled aluminum sheet, the steps comprising subjecting said sheet material to at least one.

finishing pass between highly polished rolls maintained at difierent temperatures, the roll having the lower temperature being in contact with the bright side of the sheet, and feeding the sheet to said rolls at an angle to the center line of the pass and from the side of said center line opposite to that where the roll producing the bright surface is located.

5. In a method of producing one side bright aluminum sheet material of greater thickness than foil, the sheet being characterized by one surface having substantially the brightness of flat rolled aluminum sheet, the steps comprising "subjecting said sheet material to a plurality of finishing passes between highly polished lubricated rolls maintained at a temperature differential of about 50 F., the roll having the lower temperature being in contact with the bright side of the sheet, and feeding the sheet to said rolls at an angle to the center line of the pass and from the side of said center line opposite to that where the roll producing the bright surface is located.

6. In a method of producing cold rolled one side bright coiled aluminum strip material of greater thickness than foil, the strip being characterized by one surface having substantially the brightness of flat rolled aluminum sheet, the steps comprising heating the cold rolled strip to at least 100 F. but not over F., feeding said strip to highly polished lubricated rolls at an angle to the center line of the pass, said rolls being maintained at different temperatures, the roll at the lower temperature being in contact with the bright side of the strip and on the opposite side of thecenter line of the pass from which the strip is introduced to the rolls.

'7. In a method of producing cold rolled one side bright coiled aluminum strip material of greater thickness than foil, the strip being characterized by one bright surface having substan tially the brightness of flat rolled aluminum sheet,

the steps comprising subjecting said strip at a temperature of 100 to 140 F. to at least one finishing pass between highly polished rolls maintained at a temperature differentialof about 50 F., the temperature of the warmest roll not exthat where the roll producing the bright surface is located.

8. In a method of producing cold rolled one side bright coiled aluminum strip material of greater thickness than foil, the strip being char- "tween highly polished lubricated rolls maintained acterized by one bright surface having substantially the brightness of flat rolled aluminum sheet, the steps comprising heating said strip to between 100 and 140 F., subjecting said heated strip to not more than four finishing passes between highly polished rolls maintained at a temperature differential of about 50 F., the temperature of the warmest roll not exceeding 200 F., the roll having the lower temperature being in contact with the bright side of the strip, and feeding the strip to said rolls at an angle to the center line of the pass and from the side of said center line opposite to that where the roll producing the bright surface is located, said feeding being at such a rate that the strip is rolled at 100 to 150 feet per minute.

9. In a method of producing cold rolled one side bright coiled aluminum strip material of greater thickness than foil in a temper between fully worked and fully annealed sheet, the strip being characterized by one surface having substantially the brightness of flat rolled aluminum sheet, the steps comprising subjecting said strip at an intermediate gauge between hot rolled plate and the finish thickness to at least one pass beat a temperature differential of 50 F., the temperature of the warmest roll not exceeding about 200 F., the roll having the lower temperature being in contact with the bright side of the strip, and feeding the strip at an angle to the center line of the pass and from the side of said center line opposite to that where the roll producing the bright surface is located, annealing said strip at any eflective softening temperature up to 650 F.; 3 and repeating the aforesaid rolling operation as a finishing pass without reversing the side of the strip in respect to the location of the roll producing the bright surface.

10. As a new strip mill product, a cold rolled aluminum strip which on emergence from the.

last roll pass is characterized by a specular reflective surface on one side thereof and a diffuse reflective surface on the opposite side.

11. As a new strip mill product, a cold rolled aluminum strip having a temper between that of fully worked strip and fully annealed strip of the same composition and characterized by a specular reflective surface on one side thereof and a diffuse reflective surface on the opposite side.

ROBERT WILLIAM GR. 

